The half-life of strontium- 90 is 28 years. How long will it take a $60 \mathrm{mg}$ sample to decay to a mass of $15 \mathrm{mg}$ ?

Step 1 ：The half-life of a substance is the time it takes for half of the substance to decay. In this case, we are given that the half-life of strontium-90 is 28 years. This means that every 28 years, the mass of strontium-90 will be halved.

Step 2 ：We start with a 60mg sample and want to know how long it will take to decay to 15mg. This is a quarter of the original mass, which means it will take two half-lives for this to occur.

Step 3 ：Therefore, we can calculate the time it will take for the sample to decay to 15mg by multiplying the half-life of strontium-90 by 2.

Step 4 ：Let's denote the half-life as \(half\_life = 28\)

Step 5 ：Then, the time it takes to decay is \(time\_to\_decay = half\_life \times 2 = 56\)

Step 6 ：Final Answer: It will take \(\boxed{56}\) years for a 60mg sample of strontium-90 to decay to a mass of 15mg.